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Narrowband Interference in OFDM System
Khaizuran Abdullah, Nadiatul Fatiha Hussin and Saidatul Izyanie Kamarudin
1.0 INTRODUCTION
The nature of future wireless applications demands high data rates. The idea of
multi-carrier transmission has surfaced recently to be used for combating the hostility of
wireless channel and providing high data rate communications. OFDM is a special form
of multi-carrier transmission where all the subcarriers are orthogonal to each other.
OFDM promises a high user data rate transmission capability at a reasonable
complexity and precision. Future telecommunication systems must be spectrally
efficient to support a number of high data rate users. OFDM uses the available spectrum
very efficiently which is very useful for multimedia communications. For all of the
above reasons, OFDM has already been accepted by many of the future generationsystems [1]. OFDM is a special case of multicarrier transmission, where a single data
stream is transmitted over a number of lower rate subcarriers [2]. One of the main
reasons to use OFDM is to increase the robustness against frequency selective fading or
narrowband interference. In a multicarrier system, only a small percentage of the
subcarriers will be affected by the interference. In the mid-1960s, the idea of using
parallel data transmission and frequency division multiplexing was published. It started
with division of the total signal frequency band into N non-overlapping frequency sub-
channels. Then, each sub-channel will be modulated by separate symbol and then the N
sub-channels are frequency-multiplexed. This led to inefficient use of bandwidth. Then
the idea to use parallel data and frequency division multiplexing with overlapping sub-
channels is introduced [2]. This may save almost 50% of the bandwidth. To make the
sub-channels overlapping it is important to make sure the orthogonality between the
different modulated carriers.
Researches have been done multicarrier transmission based on orthogonal
frequency, like in 1971 Weinstein and Ebert applied the discrete Fourier transform to
parallel data transmission system in the process of modulation and demodulation [2]. In
1960s, the OFDM system is used in several military systems such as KATHRYN which
used up to 34 parallel low-rate phase-modulated channels [2]. In the 1980s, OFDM was
focused for high-speed modems, digital mobile communications, and high density
recording. Furthermore, in 1990s, more aggressive development has been done forwideband data communication over mobile radio FM channels, asymmetric digital
subscriber line (ADSL), digital audio and more. OFDM has become the physical layer
choice for many wireless communication systems because its design ability to operate
in unlicensed spectrum [11] is featured in the current wireless local area network
(WLAN) and wireless metropolitan area network (WMAN). These systems however
must share spectrum with other unlicensed systems which produces narrowband
interference in WLAN and WMAN.
The important issue need to be considered is the effect of radio frequency
interference (RFI) to the OFDM system. RFI can be categorized into two types which
are narrowband interference and broadband interference [10].
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Disadvantages of OFDM:
It is more sensitive to frequency offset and phase noise.
It has relatively large peak-to-average power ratio, which tend to reduce powerefficiency of the RF amplifier. [2]
4.0 Block Diagram of OFDM
Serial I ITx Data
Q Q
Transmitter
Serial RxData
I I
Q Q
Receiver
Figure 4.1 Block diagram of OFDM
The explanation of each of block diagram of Figure 4.1 is discussed below:
4.1 SERIAL TO PARALLEL CONVERSION
In OFDM, data transmitted in the form of serial data stream. Thus, this Serial to
Parallel Conversion stage converts the input serial bit stream to the data transmitted ineach OFDM symbol. The data allocated to each symbol depends on the modulation
scheme used and the number of subcarriers. Examples of modulation scheme are 16-
QAM and 8-PSK. In 16-QAM each carrier will carry 4 bits of data. The data will be
converted back to the original serial data stream at the receiver [2].
4.2 SUBCARRIER MODULATION
Once each subcarrier has been allocated bits for transmission, they are mapped
using a modulation scheme to a subcarrier amplitude and phase, which is represented by
a complex In-phase and Quadrature-phase (IQ) vector. For example 16-QAM mapped 4
Serial
To
Parallel
Modulatio
n Slicer
RF
Demodulat
or
FFT
RF
Modulato
r
IFFTModulatio
n
Mapping
Channel
Paralle
l to
Serial
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bits for each symbol. Each combination for the 4 bits of data corresponds to a unique IQ
vector.
In the receiver side, the receiver maps the receive IQ vector back to the data
word by performing subcarrier demodulation. Each of the IQ points is blurred in
location due to the channel noise. The receiver has to estimate the nearest originaltransmission IQ vector [2].
4.3 FREQUENCY TO TIME DOMAIN CONVERSION
After the data subcarriers are set to an amplitude and phase, all unused
subcarriers are set to zero. This arranges the OFDM signal in the frequency domain.
Then, inverse fast Fourier transform (IFFT) is used to convert the signal into time
domain in order to be transmitted [2].
4.4 RF MODULATION
A baseband signal generated from OFDM modulator must be mixed up to the
required transmission frequency. This can be implemented by two ways: analog
techniques or digital up convertor. Both techniques complete the same operation.
However the performance of the digital modulation will tend to be more accurate due to
improve matching between the processing of the In-phase and Quadrature-phase
channels, and the phase accuracy of the digital IQ modulator [2].
4.5 IFFT and FFT
Inverse Fast Fourier Transform (IFFT) is used to generate OFDM symbols. The
data bit is represented in frequency domain and it is used in transmitter to handle the
process since IFFT convert signal from frequency domain to time domain. IFFT is
defined as the following equation:
0 (2.1)
FFT function can be used to find IFFT function with the changes in certain
properties. The function below is Twiddle factor for Fast Fourier Transform (FFT):
(2.2)
The Twiddle factor for IFFT can be found by adding a scaling factor of 1/N and
replacing twiddle factor value (Wnk) with the complex conjugate (W-nk) to the Twiddle
factor of FFT. [2]
4.5 Multiplexing of OFDM
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In general, the multiplexing schemes are based upon time, frequency and code.
Depending upon the scheme multiplexing is done before and/or after the channel coding
and/or modulation.
Figure 4.4 Multiplexing
An OFDM signal consists of a sum of subcarriers that are modulated by using
Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). IFFT will
modulate a block of input QAM values onto a number of subcarriers. In the receiver, the
subcarriers are demodulated by FFT. FFT performs the reverse operation of an IFFT.
4.5.1 Improve throughput
To improve the trough put, more than one carrier is used.
Figure 4.5 Multiple carriers used in OFDM [19]
Figure 4.5 shows the division of frequency before entering IFFT. It shows multiple
carriers which have been used in OFDM in order to improve the throughput. Thus, at
the receiver the data is not easily affected by multipath distortion [3].
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4.5.2 Subcarrier spacing
The sub-carriers are spaced at regular intervals called the sub-carrier frequency
spacing (F). The sub-carrier frequency relative to the center frequency is k F where k
is the sub-carrier number. Figure 6 shows the subcarrier spacing.
Figure 4.6 Subcarrier spacing [19]
4.5.3 Serial symbol transmission
A constellation diagram is a representation of a digital modulation scheme in the
complex plane. Each symbol will be representing in the real and imaginary axes. The
axes are often called in-phase (I axis) and the quadrature-phase (Q axis). Below are the
digital modulation overview about I and Q component of signal. Figure 7 is presentation
of signal in complex plane. Q and I is 90 degree in phase to each other as shown in
Figure 8.
Figure 4.7 Signal in complex plane
Figure 4.8 In-phase and quadrature-phase
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Figure 4.9 OFDM symbol transmission [19]
4.5.4 Parallel symbol transmission
Figure 4.10 IFFT [19]
Multiple carriers will transmit many symbols in parallel. Each carrier may have
different modulations BPSK, QPSK 64QAM. Constellation display which consistof composite of all OFDM sub-carrier symbol
Figure 4.11 Constellation Display which is composite of all OFDM Sub-carriers [19]
Figure 4.11 shows the individual subcarrier which is orthogonal to each other.
The orthogonality may allow each subcarrier overlapping between symbols transmitted.
The transmitted symbol can be retrieved back at the receiver by using FFT.
4.6 Subcarrier
Generation of subcarrier by IFFT
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Figure 4.13 A frequency domain signal comes out as a time domain signal out of IFFT
In OFDM, subcarriers are orthogonal to each other. The orthogonality allows
simultaneous transmission on a lot of subcarriers in a tight frequency space without
interference from each other [4].
Figure 4.14 Simultaneous transmissions on a lot of subcarriers in a tight frequency
space
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Figure 4.15 Signal representations in (a) time domain and (b) frequency domain
Forward FFT takes a random signal, multiplies it successively by complex
exponentials over the range of frequencies, sums each product and plot the results as a
coefficient of that frequency. The coefficients are called a spectrum and represent how
much of that frequency is present in the input signal. The result of the FFT in common
understanding is a frequency domain signal.
FFT in sinusoids,
(2.3)
x(n) the coefficients of the sines and cosines of frequency 2k / N
k the index of the frequencies over the N frequencies
n the time index
In equation (2.3), x(k) is the value of the spectrum for the kth frequency and x(n)
is the value of the signal at time n. The IFFT takes this spectrum and converts the whole
thing back to time domain signal by again successively multiplying it by a range of
sinusoids.
The equation for IFFT is,
(2.4)
In order to ensure that the subcarrier frequencies do not interfere with each other
during detection, the subcarriers are selected from a set of orthogonal signals. This
means that the spectral peak of each subcarrier overlaps with the spectral nulls of the
remaining carriers. This can be achieved by ensuring that the individual subcarriers are
spaced by an integer multiple of the inverse of the symbol duration. Highest spectral
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efficiency will be achieved when the individual carriers/subcarriers are placed precisely
one symbol duration apart [5].
One of the ways to implement OFDM is using the Discrete Fourier Transform
pair (IDFT/DFT). The implementation of an Inverse Discrete Fourier Transform (IDFT)
which corresponds to OFDM modulation is shown in Figure 2.16;
Figure 4.16 OFDM modulation using IFFT [5]
WhereN = Size of DFT
K = Frequency Index
N = Time Index
X(k) = Data corresponding to kth subcarrier
X(n) = Time domain signal
5.0 Narrowband Interference
Narrowband Interference is commonly found in communication system. Sources
of narrowband interference potentially come from other sources with frequency bandsbelow 5 MHz. The unintentional transmission such as radio and TV stations, pager
transmitters, and cell phones are the source of narrowband interference. In addition, the
source also comes from other unlicensed controlled systems such as cordless phones
and remote controlled of the garage [13]. Their spectrum resides within OFDM
spectrum, thus, degraded the performance of the OFDM system.
The important issue need to be considered is the effect of radio frequency
interference (RFI) to the OFDM system. RFI can be classified into two types which are
narrowband interference and broadband interference [10].
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The result of this project is divided into two main parts. The first part is
narrowband interference in OFDM system spectrum. The second part is BER plot of
OFDM with narrowband interference.
6.1 Narrowband interference in OFDM
-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25-50
-40
-30
-20
-10
0
10
Normalized Frequency, fn
BPFSp
ec.,
dB
OFDM spectrum
Figure 6.1 OFDM spectrum
Figure 6.1 shows the OFDM spectrum of 16-QAM modulation scheme OFDM
signal being transmitted.
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-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25-50
-40
-30
-20
-10
0
10
Normalize d Frequency, fn
AWGN
OFDM with AWGN
Figure 6.2 OFDM with AWGN
Figure 6.2 displays the OFDM spectrum after the transmitted signal going
through the AWGN channel. AWGN will add noise to the signal being transmitted.
-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25-50
-40
-30
-20
-10
0
10
20
30
40
50
Normalize d Frequency, fn
OFDMwithNBI
OFDM with NBI
Figure 6.3 Narrowband interference in OFDM spectrum
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The figure above shows the spectrum of narrowband interference in OFDM
system. The signal now consists of the AWGN and narrowband interference. Since the
narrowband interference is a single-tone sinusoid, thus it will only affect some part of
the OFDM spectrum. The subcarrier adjacent to the narrowband interference will be
affected.
6.2 BER performance of OFDM using 16-QAM
0 1 2 3 4 5 6 7 8 9 1010
-6
10
-5
10-4
10-3
10-2
10-1
100
SNR(dB)
BitErrorRate
BER of OFDM 16QAM with NBI
X: 3
Y: 0.1025
X: 3
Y: 0.02288
OFDM with narrowband interference
OFDM without narrowband interference
Figure 6.4 Comparison of BER OFDM 16-QAM with NBI and without NBI
The figure 6.4 shows the plotting of BER of OFDM 16QAM with narrowband
interference (NBI). From the graph it can be seen that the performance of OFDM
system is degraded due to the presence of NBI. For example, at 3 dB of SNR, the BER
without NBI is 0.02288 whereas the BER with NBI is 0.1025. The error for the first isabout 23 out of 1000. On the other hand, the second is 100 out of 1000. Therefore, more
errors were found when there is NBI.
6.3 BER performance of OFDM using 4-QAM and 64-QAM
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0 1 2 3 4 5 6 7 8 9 1010
-6
10-5
10-4
10-3
10-2
10-1
SNR(dB)
BitErrorRate
BERof OFDM4-QAMwithNBI
X: 3
Y: 0.02288
OFDMwithnarrowbandinterference
OFDMwithout narrowbandinterference
Figure 6.5 Comparison of BER OFDM 4-QAM with NBI and without NBI
Figure 6.5 illustrates the plotting of BER of OFDM 4-QAM with NBI and
without NBI. In comparison, both of the BER are almost have the same performance.
When SNR is equal to 3dB BER is equal to 0.02288. That is about 20 out of 1000
errors. Thus, the performance of 4-QAM is not so much affected by NBI.
0 1 2 3 4 5 6 7 8 9 1010
-6
10-5
10-4
10-3
10-2
10-1
100
SNR(dB)
BitErrorRate
BER of OFDM 64-QAM with NBI
X: 3
Y: 0.2033
X: 3
Y: 0.02288
OFDM with narrowband interference
OFDM without narrowband interference
Figure 6.6 Comparison of BER OFDM 64-QAM with NBI and without NBI
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Figure 6.6 displays BER plot of 64-QAM OFDM with NBI and without NBI. It
indicates that the performance of 64-QAM OFDM is degraded due to NBI. For instance,
when SNR is equal to 3dB BER is 0.2033. The error found in 64-QAM is about 200 out
of 1000. In contrast, there is only 20 errors out of 1000 in 64-QAM without NBI.
In comparison with section 6.2, more errors are present in 64-QAM and lesserrors in 4-QAM. This is due to bit involved in the modulation. 4-QAM has less error
but its data rates is lower compare to 16-QAM. Oppositely, 64-QAM has higher data
rates but it is susceptible to more errors. 64-QAM is more complex and requires more
bandwidth but, more errors generated. This is referred to BER Figure 4.6 for 64-QAM.
That is why is this project 16-QAM has been selected since it gives an intermediate
result of QAM modulation between 4-QAM and 64-QAM. Besides, 16-QAM is widely
used in OFDMs applications as one of the standard modulation scheme. Examples of
the application are Digital Video Broadcasting (DVB), Digital Multimedia Broadcast
(DMB) and IEEE802.11 [18], and also, WiFi and WiMAX.
7.0 CONCLUSION
This chapter discussed and concluded what have been fulfilled from the project
by reviewing the objectives and concept of the project acquired. The initial steps in
conducting this project is by carry out a comprehensive research and investigation. The
first part is about the understanding of basic Orthogonal Frequency Division
Multiplexing (OFDM) and investigating the effect of Narrowband interference on the
system. MATLAB coding has been generated in order to understand both of the
concepts. The methodology of the implementation of the project can be achieved
through understanding the basic of OFDM system, how the narrowband interference
will affect the system, and coding of MATLAB.
7.1 FUTURE WORK
For the future work, the broadband interference investigation could be done to
improve the communication system. This is because not only narrowband interference
presents in the OFDM system but also broadband. The difference is broadband
interference affect almost the entire OFDM spectrum. Thus, broadband interference is
significant also.
Besides, the studies of mitigation techniques of narrowband interference in
OFDM could be done in future to improve the performance of OFDM system. Example
of the mitigation techniques in [13] are frequency domain cancellation, excision
filtering and etc. Last but not least is the peak-to-average-power ratio problem which
can degrade the performance of OFDM system. Thus, the study of it could be done in
future to investigate its effect in OFDM system.
8.0 REFERENCE
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[1] Marchetti, N., Rahman, M. I., & Kumar, S. OFDM: Principles and Challenges,
New Directions in Wireless Communications Research, Springer
Science+Business Media. pp. 29-62, 2009.
[2] R. V. Nee, & R. Prasad, OFDM for Wireless Multimedia Communication,
London: Artech House, 2000.
[3] Introduction to Orthogonal Frequency Division Multiplex Technology, 2004,
Retrieved from www.keithley.com:
http://www.ieee.li/pdf/viewgraphs/introduction_orthogonal_frequency_division
_multiplex.pdf.
[4] Orthogonal Frequency Division Multiplexing(OFDM), 2004, Retrieved from
http://www.complextoreal.com/chapters/ofdm2.pdf.
[5] BIBLIOGRAPHY \l 1033 Navalekar, A., Design of a High Data Rate Audio
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[6] Sklar, B.,Digital Communications Fundamentals and Application, New Jersy:
Prentice Hall, 2001.
[7] Marey, M., & Steendam, H., Analysis of the narrowband interference effect on
OFDM timing synchronization, IEEE Transaction on signal processing, pp.4558-4566, 2007.
[8] Frein, C. d., Flanagan, M., & Fagan, A., "OFDM Narrowband Interference
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[9] QAM and QPSK , n.d, Retrieved from
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[10] Radio-Sky Journal. (2001, March). Retrieved fromhttp://www.radiosky.com/journal0901.html
[11] A. J. Coulson, "Bit Error Rate Performance of OFDM in Narrowband
Interference with Excision Filtering", IEEE Transactions on Wireless Com-
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[12] A. S. Osman, (December 2006). BER Performance Study of Orthogonal
Frequency Division Multiplexing (OFDM)
[13] K. Abdullah, (August 2009). Interference Mitigation Techniques for Wireless
OFDM. pp. 81-85
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[14] J. G. Proakis and M. Salehi, Communication Systems Engineering, Second ed.,
New Jersey: Prentice Hall, 2002.
[15] D Zhang, P. Fan and Z. Cao "A Novel Narrowband Interference Canceller for
OFDM systems," Wireless Communications and Networking Conference, pp.1426-1430, 2004.
[16] Z. Zhang, S.C. Chan, and H. Cheng, "Robust adaptive channel estimation of
OFDM systems in time-varying narrowband interference," 2005.
[17] K. Abdullah, N.A. Hinai, A.Z. Sadik and Z. M. Hussain, "Circular 16-QAM
Modulation Scheme for Wavelet and Fourier Based OFDM Systems", The 5th
IEEE GCC Conference, Kuwait, March 2009.
[18] K. Abdullah, A. F. Ismail, W. Hashimand Z. M. Hussain, An Optimal Circular
16-QAM Modulation Technique for Wavelet and Fourier Based OFDM,International Conference on Computer and Communication Engineering, Kuala
Lumpur, May 2010
[19] (2004). Retrieved from OFDM/MIMO Master Class Understanding the physical
layer principles of WLAN,WiMAX and LTE.